1. Field of the Invention
This invention relates to a bracket that is attached to a tooth for orthodontic treatment, and particularly concerns a bracket having a ball hook that is integrally formed with a tiewing of the bracket.
2. Description of the Related Art
Orthodontic treatment involves movement of malpositioned teeth to desired positions for correct occlusion and improved aesthetics. During treatment, small slotted bodies known as brackets are affixed to the teeth and an archwire is secured in the slot of each bracket. The archwire serves as a track to guide movement of the teeth toward desired positions.
In some instances, a small hook is affixed to certain brackets for movement of the teeth in particular directions. One end of an elongated elastic member is connected to the hook, and the other end is connected to a hook mounted elsewhere such as on another bracket, on an archwire, or on another orthodontic appliance in the oral cavity. The elastic member is under sufficient tension when in place to provide a resilient force that urges the teeth in desired directions.
U.S. Pat. No. 5,125,831, assigned to the assignee of the present invention, describes an orthodontic bracket having four tiewings and a hook that is integrally connected to one of the tiewings. (Tiewings are tiny wings on the bracket that function to grasp an O-ring or a ligature wire that connects the archwire to the bracket.) The hook has opposed notches to provide connection to an elastic member extending in a direction away from either notch. The bracket described in U.S. Pat. No. 5,125,831 is similar to a bracket presently manufactured in a machining operation and sold by the assignee of the present invention under the trademark "UNI-TWIN".
The hook of the bracket shown in U.S. Pat. No. 5,125,831 has a generally flat configuration when viewed toward the mesial side of the bracket (i.e., toward the side of the bracket facing midline of the arch) or the distal side of the bracket (i.e., toward the side of the bracket facing away from the midline of the arch). However, other brackets having hooks termed ball hooks are also known; such hooks include a shank with an enlarged, somewhat spherical head, and the shank is elongated and has a generally circular configuration in transverse cross section. Optionally, the shank of such ball hooks can be bent by the orthodontist in a desired direction to decrease the likelihood that the elastic member does not unintentionally detach from the hook during treatment. Ball hooks are preferred by some orthodontists over flat hooks because the enlarged ball head provides better retention of the elastic member in some instances.
As can be appreciated, the strength of the ball hook, and especially of the shank, is a matter of significant importance. For example, if the shank breaks from the tiewing as the shank is bent to shift the hook to a certain orientation, the entire bracket must be detached from the tooth and replaced with a new assembly. Such a procedure is time consuming and a nuisance to both the orthodontist and the patient.
In some instances, the shanks of ball hooks are connected to a tiewing of the bracket by a brazing operation. However, brackets having a hook brazed to a tiewing are not entirely satisfactory, since the brazing is relatively expensive, and careful alignment of the small parts during brazing is needed. Also, errors during brazing operation may not be readily apparent, but may result in a weak joint between the shank of the hook and the tiewing such that the shank detaches from the bracket when bent.
In other instances, ball hooks are made integrally with the brackets when the brackets (including the hooks) are manufactured in a casting process or in a sintering operation. Cast and sintered brackets having integral ball hooks are generally less expensive to manufacture than brackets having brazed hooks, but there is a possibility that the shank of the hook may have insufficient strength as a result of the casting or sintering operation, due in part to the relatively small transverse cross-sectional areas of the shank. For example, when brackets are made by a metal injection molding operation, the metal powder may not flow and/or pack in a satisfactory manner in tiny cavities of the die that correspond to the shank of the resultant bracket hook, with the result that the strength of the shank is diminished.